The requirements for high areal recording density impose increasingly greater requirements on thin film magnetic recording media in terms of coercivity, remanent squareness, low medium noise and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high density magnetic rigid disk medium.
The linear recording density can be increased by increasing the coercivity of the magnetic recording medium. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically noncoupled grains. Medium noise, typically measured as the signal to noise ratio (SNR), is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Therefore, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and typically comprises a non-magnetic substrate 10 having sequentially deposited thereon an underlayer 11, such as chromium (Cr) or a Cr-alloy, a magnetic layer 12, typically comprising a cobalt (Co) alloy, a protective overcoat 13, typically containing carbon, and a lubricant topcoat 14. Underlayer 11, magnetic layer 12 and protective overcoat 13 are typically deposited by sputtering techniques. The Co alloy magnetic layer normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy underlayer.
It is recognized that the relevant magnetic properties, such as coercivity (Hc), magnetic remanence (Mr) and coercive squareness (S*), which are critical to the performance of a Co base alloy magnetic thin film, depend primarily on the microstructure of the magnetic layer which, in turn, is influenced by the underlayer on which it is deposited. Conventional underlayers include Cr, molybdenum (Mo), tungsten (W), titanium (Ti), chromium-vanadium (CrV) as well as Cr alloyed with various substitutional elements. It is recognized that underlayers having a fine grain structure are highly desirable, particularly for growing fine grains of hexagonal close packed (HCP) Co alloys deposited thereon.
It has been reported that nickel-aluminum (NiAl) films on a glass substrate exhibit a grain size which is smaller than similarly deposited Cr films which are the underlayer of choice in producing conventional magnetic recording media. Li-Lien Lee et al., "NiAl Underlayers For CoCrTa Magnetic Thin Films", IEEE Transactions on Magnetics, Vol. 30, No. 6, pp. 3951-3953, 1994.
Accordingly, NiAl thin films are potential candidates as underlayers for magnetic recording media for high density longitudinal magnetic recording. Such a magnetic recording medium is schematically illustrated in FIG. 2 and comprises glass substrate 20, NiAl underlayer 21 and cobalt alloy magnetic layer 22 (protective overcoat and lubricant topcoat omitted for illustrative convenience). However, it was found that the coercivity of a magnetic recording medium comprising an NiAl underlayer, such as that depicted in the FIG. 2, is too low for high density recording, e.g. about 2,000 oersteds (Oe).
Lee et al. subsequently reported that the coercivity of a magnetic recording medium comprising a glass substrate and an NiAl underlayer can be significantly enhanced by depositing a plurality of underlayers containing alternative NiAl and Cr layers rather than a single NiAl underlayer. Li-Lien Lee et al., "Effects of Cr Intermediate Layers on CoCrPt Thin Film Media on NiAl Underlayers," Vol. 31, No. 6, Nov. 1995, pp. 2728-2730. Such a magnetic recording medium comprising an alternative NiAl layer and Cr layer composite structure is schematically illustrated in FIG. 3.
Adverting to FIG. 3, the depicted magnetic recording medium comprises glass substrate 30 having sequentially formed thereon Cr sub-underlayer 31, NiAl underlayer 32, Cr intermediate layer 33, and Co alloy magnetic layer 34 (protective overcoat and lubricant topcoat omitted for illustrative convenience). It was found, however, that such a magnetic recording medium is characterized by an underlayer structure exhibiting a (110)-dominant crystallographic orientation which does not induce the preferred (1120)-dominant crystallographic orientation in the subsequently deposited Co alloy magnetic layer and is believed to contribute to increased media noise. Li-Lien Lee et al. were able to obtain an underlayer exhibiting a (200)-dominant crystallographic orientation by initially depositing a Cr sub-underlayer directly on the non-magnetic substrate at an undesirably high temperature of about 260.degree. C. using radio frequency (RF) sputtering. However, deposition of a Cr sub-underlayer at such an elevated temperature undesirably results in significantly larger grains than grains resulting from deposition at lower temperatures, e.g. approximating room temperature (25.degree. C.). The formation of such larger grains is inconsistent with the very reason for employing NiAl as an underlayer. On the other hand, it is very difficult to obtain a Cr (200)-dominant crystallographic orientation, even at elevated temperature such as 260.degree. C., on glass and glass ceramic substrates using direct current (DC) magnetron sputtering, which is widely employed in the magnetic recording media industry.
Li-Lien Lee et al. recognized the undesirability of resorting to high deposition temperatures to obtain a (200)-dominant crystallographic orientation in the underlayer structure. It was subsequently reported that an underlayer structure exhibiting a (200)-dominant crystallographic orientation was obtained by depositing a magnesium oxide (MgO) seedlayer using radio frequency (RF) sputtering. Li-Lien Lee et al., "Seed layer induced (002) crystallographic texture in NiAl underlayers," J. Appl. Phys. 79 (8), Apr. 15, 1996, pp. 4902-4904; and David E. Laughlin et al., "The Control and Characterization of the Crystallographic Texture of the Longitudinal Thin Film Recording Media," IEEE Transactions on Magnetics, Vol. 32, No. 5, September 1996, pp. 3632-3637. Such a magnetic recording medium comprising a MgO seedlayer and NiAl underlayer is schematically illustrated in FIG. 4 and comprises MgO seedlayer 41 deposited on substrate 40, NiAl underlayer 42 deposited on MgO seedlayer 41, and Co alloy magnetic layer 43 deposited on NiAl underlayer 42 (protective overcoat and lubricant topcoat omitted for illustrative convenience). Such a magnetic recording medium, however is not commercially viable from an economic standpoint, because sputtering systems in place throughout the industry making magnetic recording media with the conventional structure of magnetic layers epitaxially formed on underlayers are based upon direct current (DC) sputtering. Accordingly, RF sputtering an MgO seedlayer is not economically viable.
On the other hand, the objective of having a (200)-dominant crystallographic orientation in the underlayers is to induce (1120) crystallographic orientation in the Co alloy layers. Even through media comprising an MgO seedlayer and NiAl underlayer have a (200)-dominant crystallographic orientation in the underlayer, it does not have a (1120)-dominant crystallographic orientation in the Co alloy layer, according to Laughlin et al., "The Control and Characterization of the Crystallographic Texture of the Longitudinal Thin Film Recording Media," IEEE Transaction on Magnetics, Vol. 32, No. 5, September 1996, p. 3634. Laughlin et al. reported that the grain-to-grain epitaxial relationship between the (002) NiAl and the CoCrPt film is found to be [1011] CoCrPt//[001] NiAl, and (1210) CoCrPt//(100) NiAl, or (1210) CoCrPt//(010) NiAl. In other words, Laughlin et al. reported that there is no (1120) CoCrPt//(200) NiAl epitaxial relationship found in the films with MgO seedlayers and NiAl underlayers. Rather, (200) orientation is identical with (002) orientation. When an FeAl underlayer is used instead of NiAl, it was reported that the (200) FeAl underlayer can only induce a weak (1120) textured CoCrPt by employing a MgO seedlayer or a (200) textured Cr seedlayer. Li-Lien Lee et al., "FeAl underlayers for CoCrPt thin film longitudinal media," CC-01, 41st Annual Conference on Magnetism and Magnetic Materials, Atlanta, Ga., Nov. 12-15, 1996.
In copending application Ser. No. 08/945,084, filed on Oct. 17, 1997 now U.S. Pat. No. 6,010,785, a magnetic recording medium is disclosed which comprises a surface oxidized NiP seedlayer, a Cr sub-underlayer thereon, a NiAl or FeAl underlayer formed on the sub-underlayer and a Cr or Cr alloy intermediate layer formed on the underlayer.
There exists a continuing need for high areal density magnetic recording media exhibiting a high signal to noise ratio (SNR) and high coercivity. There is a further continuing need for such magnetic recording media which exhibit improved overwrite properties and reduced modulation of magnetic properties, and are characterized by the absence of any substantial superlinear noise behavior at high areal recording densities.